HashSet: membership and set maths

A `HashSet<T>` is a `HashMap` with no values — just keys. Inserting a duplicate returns `false` and changes nothing, which makes deduplication a one-liner.

Membership tests are constant time on average, so a set is the right answer whenever you find yourself scanning a `Vec` with `contains` inside a loop.

Unions, intersections and differences are built in and return iterators, so nothing is allocated until you `collect`.

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